This invention relates to a high capacity, channel reuse, mobile, radiotelephone system providing both conventional subscriber radiotelephone service and dispatch type radiotelephone service.
Prior radiotelephone systems usually allot a separate duplex channel frequency pair per mobile party in each mobile service area (typically a city) for conventional radiotelephone subscriber messages. Thus, such a frequency pair was required for each individual mobile unit in a given area involved in a conference call. Similarly, in current high capacity cellular radiotelephone systems, a separate channel is required for each mobile unit involved either in an individual call or in a conference call, but that is a less significant penalty in the cellular channel reuse types of systems than in other radiotelephone systems because of the spectrally efficient nature of the cellular plan.
In a dispatch system, a message may need to be sent either in a single mobile unit or to a group of mobile units of a fleet; and in the latter case, the group may have different sizes at different times. The mobile units of a group in a fleet of units, such as the units in a taxi service, a sales force, or a utility service crew, may also be scattered in widely separated geographical locations. The present invention is primarily concerned with dispatch service of the latter type in which a fleet call can be made to multiple units and, in particular, with a system providing such service which is also able to provide the conventional, nondispatch, radiotelephone service.
One common type of radio dispatch system utilizes a single channel for a given fleet group, and mobile units listen to all messages broadcast to the fleet. One mobile unit user wishing to respond uses a push-to-talk (PTT) switch to activate the unit transmitter on the same channel. However, that dispatch channel is unavailable to other users either for dispatch service or for any other purposes throughout the entire mobile service area.
In an A. R. Vallarino et al. U.S. Pat. No. 2,685,642, a cellular type of dispatching arrangement is depicted in which adjacent cells utilize different frequency channels of a limited channel set, but it is possible to reuse channel sets in different nonadjacent parts of the same service area. In this case, a mobile unit user must be familiar with the boundaries of the different cells and advise the fleet dispatcher when such user is crossing into a new cell. Neither nondispatch nor fleet-call operations are considered.
The mixed traffic mobile radiotelephone system of the A. Rimbach, Jr., U.S. Pat. No. 3,786,199 contemplates combined telephone and dispatch service in the same frequency channel environment. However, this patent deals with the situation wherein a dispatcher needs to address a single unit of a fleet and does not deal with the fleet-call situation wherein a dispatcher needs to address multiple mobile unit users in a single call.
Otherwise, in cellular, high capacity, channel reuse systems, it has usually been considered that each listening and responding unit of a fleet must have its own duplex channel assignment during a dispatcher's fleet call. This type of arrangement thus requires a different duplex channel per mobile unit within any given cluster of cells of the system. That tends to require many channels during a fleet call. It is also a fact that conference bridging arrangements are required to enable the dispatcher and the fleet to listen to all calls, and the cost of such conferencing equipment puts a limit on the size of any fleet which can be economically interconnected at any one time. Conferencing was used because it has been found in fleet dispatching practice that it is sometimes desirable to let all or a substantial part of a fleet listen to fleet calls and all responses. For example, this enables the users of different units in the fleet to gauge their respective proximities to the point to which a dispatch is being made and decide, after a first responding unit has called in, whether or not one of them also should offer to participate.
One basic, mobile, radiotelephone, cellular, channel reuse system of the type indicated is described in the Bell System Technical Journal, Jan., 1979, Vol. 58, No. 1. Dispatch service considerations for such a system may be found in the "High Capacity Mobile Telephone System Technical Report" prepared by Bell Laboratories and filed Dec. 20, 1971 with the Federal Communications Commission in docket 18262. See, for example, pages 1-4 through 1-7 and 3-42 through 3-48 of that report.